EMBRIOLOGY

Week 1
For this presentation embriogenic age will be expressed as days or weeks from fertilisation. On this basis human pregnancy lasts about 38 weeks. Obstetricians time pregnancy from the last day of the menstrual period on the assumption that fertilisation take place 2 weeks later. For obstetricians pregnancy last 40 weeks.

Week 2
Implantation
To implant developing embryo must pass through the uterine epithelium. This occurs about 7 days after fertilisation. The trophoblast  produces human chorionic gonadotrophin (hCG) which maintains the corpus luteum of the ovary which in turn produces progestesterone which maintain early pregnancy. hCG can be detected in the maternal blood and forms the basis of the early pregnany test. Detection in urine is less sensitive.

Week 3
Gastrulation
The inner cell mass in the 15-16 day human embryo consist of two layers - an upper epiblast and a lower hypoblast. Gastrulation coverts this bilaminar disc into three layers. An upper ectoderm, a middle mesoderm and a lower endoderm.
Cells from each germ layer have specific fates
Ectoderm

• Epidermis, hair, nails etc
• Brain and nervous system

Mesoderm

• Muscle, cardiovascular system, bones, blood, dermis, gonads, excretory system etc

Endoderm

• Inner lining of digestive tract
• Lining of lungs
• Glands

Week 4
Nervous system development in the human embryo

• At 18 days after conceptions the embryo consistsof 3 layers of cells endoderm, mesoderm and ectoderm. Thickening of the ectoderm leads to the development of the neural plate
• The neural groove begins to develop at 20 days
• At 22 days the neural groove has closed in the middle to form the neural tube. It remains open at either end
• The anterior opening or neuropore closes day 25-26 and the posterior neuropore day 27-28. A 27-day embryo should have a completely closed neural tube. The heart is visible and has already started to beat

Week 5-8
Organogenic Period
32-day human embryo
Organ system development is well underway. This is a critical time for development of the heart, limbs, eyes, upper lip, intestines.
44-day human embryo
At this stage the embryo has completed much of its organogenesis. Still to be completed is the palate and external genitalia and brain development is ongoing.
8-week human fetus
Organogenesis is largely complete, development of the external genitalia is still incomplete. The brain is about to start at 8-week period of massive neuronal cell formation for the cerebral cortex.

Week 9-38
Fetal Period

10-week human fetus
Fetal heart beat can be monitored at this stage. Chorionic villous sampling is usually performed between 8 and 10 weeks. A small piece (villous) of the placenta is removed and cultured in the laboratory. Genetics results are ussually available in 2 weeks.

13-week human fetus
The fetus is surrounded by about 100 ml of amniotic fluid. Amniocentesis can be performed from about 13 to 18 weeks gestation. About 10-20 ml of fluid is removed, the fetal cells are separated and grown in culture and genetic results available in about 2 weeks. An ∝-fetoprotein test can be performed on the maternal blood. This protein is made by the fetus and is in higher concentrations in fetuses with neural tube defects.

16-week human fetus
An ultrasound dating scan can be given at 5-11 weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown-rump length for dating. An anomaly scan is usually performed at 16-18 weeks to look for congenital malformation. Sex of the fetus can usually be determined at this stage.

24-week human fetus
Head hair appears, the fetus already has a downy hair (lanugo), skin is coated with vernix-a waxy secretion of sebaceous glands.

38-week human fetus
Eyes reopened at about 26 weeks, at about 30 weeks skin becomes thicker and subcutaneous fat appears.